NULL Operators
To check whether a value is or is not NULL, use the constructs:
[expression IS NULL | expression IS NOT NULL]
Alternatively, use equivalent, but nonstandard, constructs:
[expression ISNULL | expression NOTNULL]
Do not write expression = NULL because NULL represents an unknown value, and two unknown values are not necessarily equal. This behavior conforms to the SQL standard.
Some applications might expect that expression = NULL returns true if expression evaluates to null. Vertica strongly recommends that these applications be modified to comply with the SQL standard.